1. Field of the Invention
The present invention relates to a method and apparatus for correcting abnormal cardiac activity by employing low energy shocks. More specifically, the present invention deals with the correction of arrhythmias such as high-rate ventricular tachycardia and ventricular fibrillation with multiple relatively low energy shocks issued in synchronism with a repeatable event in the arrhythmia.
2. Description of the Prior Art
Normal heart rhythm (sometimes called "sinus rhythm") originates in the sino-atrial (SA) node of the heart. Disorders of heart rhythm are called arrhythmias, and may arise in either the atria or the ventricles. For example, one type of arrhythmia manifests itself in the form of a rapid heart rate (known as "tachycardia") arising in the atria, which may be serious in patients with diseased hearts.
A common treatment for serious tachycardia consists, basically, in passing a pulse of electrical energy through the heart, and momentarily stopping the heart completely, after which the heart is left to start spontaneously in sinus rhythm. The instrument utilized for such treatment is known as a cardioverter. This instrument is in most respects the same as the instrument known as the defibrillator, with the major difference being that the cardioverter typically delivers its shocks in synchrony with the cardiac activity so as to avoid the vulnerable period and hence lessen the possibility of worsening the rhythm being treated. Specifically, the delivery of electrical energy in the vulnerable period (e.g., on the T-wave) could induce ventricular fibrillation.
Additionally, cardioverting pulses often are characterized by energy levels of approximately 25 to 100 joules externally, whereas the energy levels typically employed for ventricular defibrillation are in the range of 100-400 joules for external application (or 20-50 joules for internal application). For purposes of comparison, pacing is accomplished by energy levels on the order of 0.04 millijoules or less.
Ventricular fibrillation is an arrhythmia generally more difficult to break than tachycardia. As noted previously, the energies required to defibrillate generally are higher than those capable of cardioverting. And, there are several phenomena involved in ventricular defibrillation that are not adequately explained by the conventional theory that a defibrillating shock needs to be sufficiently strong to depolarize the vast majority (a critical mass) of the myocardium. For example, it has been observed that a heart not defibrillated by several shocks at a particular energy level suddenly responds to the same energy level and reverts to sinus rhythm. It further has been observed that at times a heart may not respond to high energy shocks, but effectively can be defibrillated if the energy is reduced to a lower level.
There are obvious reasons for wanting to defibrillate at low energy levels, and hence there have been efforts made to reduce the energy levels in defibrillation. Some low energy defibrillation techniques have been described, but although at times effective, such techniques are not consistently so. For example, a double pulse low energy system is described by Jan Kugelberg in the article "Ventricular Defibrillation--A New Aspect" (Stockholm, Sweden: Acta Chirurgica Scandinavica, 1967). Kugelberg discloses the use of two pulses separated by a predetermined time period, optimally 100 miliseconds. The theory is that the first pulse affects and synchronizes a small portion of the myocardium, while the second pulse then can more easily complete the job of reverting the arrhythmia. While the theory does appear to have some merit, and while experimentation has shown approximately 60% effectiveness with laboratory dogs at the optimal pulse interval of 100 milliseconds, the Kugelberg technique has not been accepted as a viable approach to defibrillation at low energies.
Another low-energy theory previously advanced involves what is termed the "protective zone" theory. This theory is that a properly timed small shock can prevent desynchronization of myocardial depolarization which otherwise could lead to ventricular fibrillation when shocks are delivered in the vulnerable period. A timed second shock, according to this theory, resynchronizes the myocardial fibers which were disrupted by the first shock. But, again, this theory has not consistently been effective in practice.
Defibrillation of the heart also has been effected by relatively low energy shocks delivered through an intravascular catheter. In this regard, it has been observed that catheter defibrillation in man often is effective using 5-15 watt seconds. See commonly owned Mirowski et al U.S. Pat. No. 3,942,536 , incorporated by reference herein. Still, there is benefit to be gained by further reducing energy levels.
It is believed that the inability to obtain consistently favorable results at low energy levels with the prior art methods can be linked to the inherent random delivery of shocks in response to the malignant arrhythmias. There is thus a need for a method and apparatus which is effective in uniformly correcting certain cardiac arrhythmias with shocks having energy levels lower than those presently employed with the techniques of the prior art. The present invention is directed toward filling such need.